Novel experimental setup for coulometric signal transduction in ion-selective electrodes

Abstract

In this research work, a simple and versatile novel experimental setup for coulometric signal transduction in ion-selective electrodes was introduced and studied. It is based on a constant potential coulometry measurement carried out using a three-electrode electrochemical cell. Conventional potassium ion-selective electrodes (K+-ISEs) and solid-contact potassium ion-selective electrodes (K+-SCISEs) with either poly(3,4-ethylenedioxythiophene) doped with polystyrene sulfonate, i.e. PEDOT:PSS, or multi-walled carbon nanotubes as solid contacts were prepared and used in this work. In the setup, a K+-ISE was connected as the reference electrode (RE). The potential difference between the RE (K+-ISE) and a PEDOT:PSS-based working electrode (WE) was kept constant by using a potentiostat. The potentiometric response of K+-ISE, which is due to change in the concentration (activity) of potassium ion in the sample, is transformed into amperometric and coulometric signals through the WE. Adding a non-selective dummy membrane to the structure of the WE and using a background electrolyte with high and constant concentration are significant steps in the regulation of the measured signal and equilibration time. These are also interesting strategies that helped ensuring the exclusive occurrence of analyte detection at the K+-ISE and signal generation at the WE despite having a single-compartment configuration. Results from electrochemical impedance spectroscopy measurements showed that the time constant is heavily influenced by the dummy membrane thickness, and that the redox capacitance of the PEDOT:PSS film has a better correlation with the electrode area than the film thickness. Sequential addition/dilution experiments showed the improvement of current and cumulated charge signals in the new setup studied in this work compared to the setup used in the original coulometric signal transduction method. Furthermore, the responses were found to be reversible and reproducible across a variety of electrodes. Results from the calibration of different WEs with different K+-ISEs used as RE presented a linear relationship between the cumulated charge and the logarithm of K+ activity. It also confirmed the direct relationship between electrode area and electrode sensitivity. Lastly, measurements carried out using human serum sample showed that the new setup for coulometric signal readout could be used for detecting small concentration changes both in synthetic and real biological samples.